It is often desirable to provide visual information to a living being, usually a person. Frequently one wishes to superimpose such visual information upon the being's view of the real world.
Such a display requires, in a form known as a folded catadioptric display, an image generator; a beam splitter, which receives the image light from the image generator and sends a fraction, designated the reflected fraction, of such image light to a reflective combiner that both allows light from the real world to pass through such combiner and reflects the image light such that both the real-world light and the image light are transmitted to the eye of the user through the beam splitter.
The beam splitter will transmit a fraction, designated the transmitted fraction, of the image light reflected from the collimator-combiner. Of course, only a fraction of the real-world light is also transmitted by the beam splitter; but attention is directed only to the image light since it is image light that tends to be critically low in intensity.
To correct for aberrations and distortions produced by the beam splitter and the combiner, a correction lens is often placed in the optical path between the image generator and the beam splitter.
The light emitted from liquid crystal displays is linearly polarized because liquid crystal displays utilize polarizers to modulate amplitude. The only cathode ray tubes which produce linearly polarized light are those which are colored with an active wave plate, such as the Tektronics Pi cell, all of the light from which is linearly polarized.
When a fold mirror is selected as the beam splitter, it is plastic or glass having, preferably, a dielectric or holographic dichromated gelatin coating. Metal coatings are unacceptable because the fold mirror must first reflect light to the combiner and then transmit light from the combiner to the eye of the user.